Recently, a secondary battery such as a lithium ion battery has been used in various fields so as to reduce environmental load. Deterioration of the secondary battery progresses while in use. To reduce the running cost of the secondary battery, it is necessary to suppress the progress of the deterioration.
Patent Document 1 and Patent Document 2 disclose an example of a charging method capable of suppressing deterioration of a secondary battery. In a method disclosed in Patent Document 1, when a voltage of a secondary battery, which is measured during charging, reaches a fully charged voltage, charging of the secondary battery is stopped, and when a voltage of the secondary battery, which is measured during discharging, is less than a constant voltage, charging of the secondary battery is restarted. In addition, Patent Document 2 discloses an example of a charging method of securing sufficient electric power so as to operate a device in consideration of a variation in demand for electric power. In Patent Document 2, a method of suppressing deterioration of the secondary battery is disclosed. In the method, a power storage device is charged in accordance with a charging target value for every hour which is set in advance in accordance with demand for electric power so as to avoid frequent charging in the vicinity of full charge.